The 1st US 321 Business was created in 1960 as a renumbering of a secondary route into Blowing Rock which was mainline US 321 until about 1954. The north half was an addition to US 221.

secondary route into Blowing Rock
1957 Watauga County

US 321 Business through Blowing Rock
1962 Watauga County

The 2nd US 321 Business was created in June 1999 (NCDOT) as re-established primary routing from ; replaced NC 155 through Dallas, High Shoals, and Lincolnton; replaced US 321 through Maiden, Conover, and Hickory (south of US 70 was also assigned as NC 155).
This is the longest non-interstate BUsiness route in North Carolina at over 30 miles.